Hydraulic fracturing is a method of using pump rate and hydraulic pressure to fracture or crack a subterranean formation. Once the crack or cracks are made, high permeability proppant, relative to the formation permeability, is pumped into the fracture to prop open the crack. When the applied pump rates and pressures are reduced or removed from the formation, the crack or fracture cannot close or heal completely because the high permeability proppant keeps the crack open. The propped crack or fracture provides a high permeability path connecting the producing wellbore to a larger formation area to enhance the production of hydrocarbons.
The development of suitable fracturing fluids is a complex art because the fluids must simultaneously meet a number of conditions. For example, they must be stable at high temperatures and/or high pump rates and shear rates which can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Various fluids have been developed, but most commercially used fracturing fluids are aqueous based liquids which have either been gelled or foamed. When the fluids are gelled, typically a polymeric gelling agent, such as a solvatable polysaccharide is used, which may or may not be crosslinked. The thickened or gelled fluid helps keep the proppants within the fluid during the fracturing operation.
While crosslinked and uncrosslinked polymers have been used in the past as gelling agents in fracturing fluids to carry or suspend solid particles in the brine, such polymers require separate breaker compositions to be injected to reduce the viscosity.
Aqueous fluids gelled with viscoelastic surfactants (VESs) are also known in the art. VES-gelled fluids have been widely used as gravel-packing, frac-packing and fracturing fluids because they exhibit excellent rheological properties and are relatively less damaging to producing formations than fluids gelled with crosslinked polymers. VES fluids are non-cake-building fluids, and thus leave no potentially damaging polymer cake residue. VES fracturing fluids offer many properties that are conducive to a well-executed frac pack or fracturing treatment. However, these fluids have little fluid loss control. The same property that makes VES fluids relatively less damaging tends to result in significantly higher fluid leakage into the reservoir matrix, which reduces the efficiency of the fluid especially during VES fracturing treatments. Fluid lost to the formation during frac pack and hydraulic fracturing operations increases the risk of slurry dehydration and premature screen-out, increases the risk of formation damage, increases the risk of fluid incompatibilities with formation fluids (e.g. emulsions), increases the volume of fluid needed to complete the treatment, and/or can cause increased hydraulic horsepower requirements.
It would thus be very desirable and important to find and use fluid loss agents for VES fracturing treatments in high permeability formations.